Screw vacuum pump having valve controlled cooling chambers

ABSTRACT

A discharge side housing  3  has a third cooling water chamber  19  communicating with a first cooling water chamber  7  of a main housing  1  through a cooling water passage  26.  The third cooling water chamber  19  is connected to a cooling water outlet pipe  27  which is connected to an inlet port  28   a  of a three-way valve  28.  The three-way valve  28  has a switching port  28   b  which can communicate with a pipe line  29  connected to the first cooling water chamber  7.  The three-way valve  28  has an outlet  28   c  connected to a pipe line  30  which communicates with a second cooling water chamber  22  of a suction side housing  2.  The second cooling water chamber  22  is connected to a cooling water discharge line  31  provided with a flow control valve  32  allowing a back pressure for a cooling water flowing thereinto.

FIELD OF THE INVENTION

The present invention relates to a screw rotor type dry vacuum pumpwhich is used, for example, in a semiconductor producing apparatus. Thevacuum pump is also sufficiently applicable to a hard process in whichsubstances produced in a reaction of a process gas accumulates in thevacuum pump.

BACKGROUND OF THE INVENTION

FIG. 5 is a longitudinal sectional view showing the construction of avacuum pump. The vacuum pump has a main housing 1, a suction sidehousing 2 attached on a right end surface of the main housing 1, adischarge side housing 3 attached on a left end surface of the mainhousing 1, and a gear housing 4 mounted in the left side of thedischarge side housing 3. On a left end portion of the gear housing 4, amotor 5 is mounted.

The main housing 1 is provided with an inner cylinder 1 a longitudinallyextending therethrough, a suction port 6 externally cummunicating withthe inner cylinder la at a right side of the inner cylinder 1 a, and acooling water chamber 7 for cooling a wall of the main housing 1.

The inner cylinder 1 a accommodates a pair of screw rotors 8 engagingwith each other (only one of them is illustrated in FIG. 5).

The suction side housing 2 is formed with two recesses in which a pairof bearing caps 9 (only one of them is illustrated in FIG. 5) arereceived to be secured therein. Each bearing cap 9 accommodates abearing 10 for rotatably supporting a shaft 8 a extending from a rightend of the screw rotor 8.

The discharge side housing 3 is formed with two recesses in which a pairof bearing caps 11 (only one of them is illustrated in FIG. 5) arereceived to be secured therein. Each bearing cap 11 accommodates abearing 12 for rotatably supporting a shaft 8 a extending from a leftend of the screw rotor 8.

Each screw rotor 8 has a tooth portion 8 b engaging with another toothportion 8 b of the opposing screw rotor 8. One of the screw rotors 8 isa driving rotor. On an outer surface of the left side shaft 8 a of thedriving rotor, a timing gear 24 is secured. In a left side of the timinggear 24, there is mounted a coupling 25 which is coupled to an outputshaft 5 a of the motor 5.

The other screw rotor 8, which is driven by the rotation of the one ofthe screw rotors 8, has another timing gear (not shown) engaging withthe former timing gear 24 and secured on a shaft 8 a attached on a leftportion of the other screw rotor 8.

The rotation of the screw rotor 8 draws in a fluid (a gas) from thesuction port 6 to discharge it from a discharge port 13.

The vacuum pump generates heat during its operation to heat itself up toa high temperature. This high temperature causes a damage of an oil sealor a lip seal for axially sealing the shaft of the screw rotor 8 or ofthe bearing supporting each end of the screw rotor 8. The hightemperature may also cause another problem such as seizing of the screwrotors 8. Therefore, a water cooling system has to be provided for thevacuum pump.

Thus, the discharge side housing 3 is provided with the discharge port13 communicating with the inner cylinder la and a cooling water chamber19 for cooling a wall of the discharge side housing 3.

The gear housing 4, which is cylindrical, has a cooling water chamber 14on an outer surface thereof, and a cooling water chamber 15 is providedon an outer surface of the motor 5.

The cooling water of the vacuum pump flows, as illustrated in FIG. 6,into the cooling water chamber 15 of the motor 5 through a cooling watersupply line 16 to cool the motor 5 and thereafter is delivered into thecooling water chamber 14 of the gear housing 4 through a connecting pipe17 to cool the gear housing 4.

The cooling water which has cooled the gear housing 4 flows through aconnecting pipe 18 into the cooling water chamber 19 of the dischargeside housing 3 to cool the discharge side housing 3 and then isdelivered into the cooling water chamber 7 of the main housing 1 througha connecting pipe 20. After the cooling water has cooled the mainhousing 1, the cooling water flows through a connecting pipe 21 into acooling water chamber 22 of the suction side housing 2 to cool thesuction side housing 2 and finally is discharged from a discharge line23.

Thus, the heat generated in operation of the vacuum pump is removed.

A dry vacuum pump used in a semiconductor producing apparatus has toaccomplish a vacuum degree of the order of 1 Pa (of 10⁻³ Torr). When agas handled by the vacuum pump is finally discharged into theatmosphere, the gas should be compressed at a compression rate of theorder of 10⁵ before the discharge, generating a large amount of heat dueto the compression.

Therefore, a cooling system with a cooling water is inevitable for thevacuum pump as well as a general vacuum pump. However, a disadvantage ofthe vacuum pump remains as described in the following.

The cooling of the main housing 1 of the dry vacuum pump cools a processgas flowing in the main housing 1, so that substances such as AlCl andNH₃Cl contained in the gas changes into solids which deposit on theinner cylinder 1 a or on the screw rotors 8. The deposits block aclearance between the pair of the screw rotors 8 and a clearance betweenthe screw rotors and the inner cylinder 1 a, interrupting the rotationof the screw rotors 8.

The vacuum pump has been used in various applications in semiconductorproducing steps. For example, the vacuum pump is used in a light processgenerally called as a clean process in which no deposits are generated.The light process, in which a conventional vacuum pump may be used withno problem, is applied in a load lock process and a sputtering process.However, deposits are generated during a process of CVD (Chemical VaporDeposition) such as Nitride or Teos for covering a thin film on a wafer.Also, deposits are generated during an Al etching process.

During the Nitride process, chemical substances react as follows.

SiH₂Cl+NH₃→Si₃N₄+NH₄Cl

During the Al etching process, chemical substances react as follows.

Al+Cl₂→AlCl₂

That is, the solid of NH₄Cl or AlCl₂ is produced.

NH₄Cl sublimes to become a gas from a solid at a temperature more than180° C. under a normal atmospheric pressure. NH₃Cl sublimes at atemperature of around 338° C.

In a vacuum state in which only an attenuated gas is existing, nodeposits are generated. Thus, a method, in which N₂ is purged into adischarge side of the screw rotor, has been proposed to prevent thegeneration of deposits in a compression stage. However, the method isstill insufficient.

Furthermore, in the semiconductor producing process including a lightprocess and a hard process, it is disadvantageous for management of theproducing process in that the two types of vacuum pumps have to beprepared for an alternate use thereof.

The present invention can be applied to a method including a N₂ purgestep and a heating step. However, in the heating step, a conventionalelectric heater is not used, but the deposit generation is limited bycontrolling heat generated by compression during operation of a vacuumpump. Furthermore, the present invention provides a dry vacuum pumpwhich is advantageously used for a light process and also for a heavyprocess with a one-touch switching operation.

SUMMARY OF THE INVENTION

For achieving the above-mentioned object, a vacuum pump according to thepresent invention includes an inner cylinder accommodating a pair ofscrew rotors engaging with each other, a suction port communicating withone side of the inner cylinder, and a discharge port communicating withanother side of the inner cylinder. The vacuum pump also includes a mainhousing having an outer wall on which a first cooling water chamber isprovided, a suction side housing attached on one end of the main housingand having an outer wall on which a second cooling water chamber isprovided, and a discharge side housing attached on another end of themain housing and having an outer wall on which a third cooling waterchamber is provided. The third cooling water chamber 19 of the dischargeside housing communicates with the first cooling water chamber of themain housing through a cooling water passage. The third cooling waterchamber of the discharge side housing has a cooling water outlet pipewhich is connected to an inlet of a three-way valve. The three-way valvehas a switching port which can communicate with the first cooling waterchamber of the main housing. The three-way valve has an outlet which isconnected to the second cooling water chamber of the suction sidehousing. The second cooling water chamber of the suction side housing isconnected to a cooling water discharge line provided with a valve.

Preferably, the valve provided in the cooling water discharge line is aflow control valve. The vacuum pump may have a temperature sensor fordetecting whether a temperature of the main housing becomes more than apredetermined value and may also have a warning device for warning ofthe open degree of the flow control valve based on a sensed signal ofthe temperature sensor.

Alternatively, the vacuum pump may have a temperature sensor fordetecting whether a temperature of the main housing becomes more than apredetermined value and may also have a control device for automaticallycontrolling the open degree of the flow control valve 32 based on asensed signal of the temperature sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view showing a dry vacuum pump according to thepresent invention;

FIG. 2 is a cross-sectional view of FIG. 1;

FIG. 3 is a sectional view taken along line 3—3 of FIG. 1;

FIG. 4 is an illustration showing a piping arrangement for a coolingwater of the vacuum pump;

FIG. 5 is a cross-sectional view showing an inner structure of aconventional vacuum pump; and

FIG. 6 is an illustration showing a piping arrangement for a coolingwater of a conventional vacuum pump.

PREFERRED EMBODIMENT

Referring to the accompanied drawings, an embodiment of the presentinvention will be discussed hereinafter.

FIG. 1 is a front view showing a dry vacuum pump according to thepresent invention; FIG. 2 is a cross-sectional view of FIG. 1; FIG. 3 isa sectional view taken along line 3—3 of FIG. 1; and FIG. 4 is anillustration showing a piping arrangement for a cooling water of thevacuum pump.

Since the dry vacuum pump has a structure based on the conventional one,the components same as those of the conventional one each have areference numeral the same as one of the conventional one and are notdiscussed again. Only features of the embodiment which are differentfrom the conventional one will be discussed hereinafter.

A third cooling water chamber 19 of a discharge side housing 3communicates with a first cooling water chamber 7 of a main housing 1through a cooling water passage 26. The third cooling water chamber 19of the discharge side housing 3 communicates with a cooling water outletpipe 27 which is connected to an inlet port 28 a of a three-way valve28.

The three-way valve 28 has a switching port 28 b which can communicatewith the first cooling water chamber 7 of the main housing 1. Thethree-way valve 28 has an outlet port 28 c which is connected to a pipeline 30 communicating with a second cooling water chamber 22 of asuction side housing 2. The second cooling water chamber 22 is connectedto a cooling water discharge line 31 provided with a flow control valve32 for controlling a back pressure of a cooling water flowing thereinto.

When the switching of the three-way valve 28 and the operation of theflow control valve 32 are carried out by hand, there is provided awarning device 41 (not shown). The warning device gives an alarm when atemperature sensor 40 (not shown) detects that a temperature of the mainhousing 1 becomes more than a predetermined value.

Meanwhile, when the switching of the three-way valve 28 and theoperation of the flow control valve 32 are automatically carried out,there is provided a control and warning device 41 for the switching ofthe three-way valve 28 and for the operation of the flow control valve32 based on signals detected by the temperature sensor 40.

Operation of thus constituted dry vacuum pump, which is applied to alight process and to a heavy process, will be discussed hereinafter.

In the light process, regarding the three-way valve 28, the switchingport 28 b is open and the inlet port 28 a is closed.

In this state, the cooling water flows sequentially through the coolingwater supply line 16, a cooling water chamber 15 of a motor 5, aconnecting pipe 17, a cooling water chamber 14 of the gear housing 4,the connecting pipe 18, and the cooling water chamber 19 of thedischarge side housing 3. Then, the cooling water further flows into thefirst cooling water chamber 7 of the main housing 1 through the coolingwater passage 26.

Thus, the main housing 1 is cooled, so that the temperature of a gasflowing through the main housing 1 becomes around 150° C.

The cooling water which has passed through the first cooling waterchamber 7 flows through a pipe passage 29 in a direction shown by anarrow F. Then, the cooling water flows through the switching port 28 bof the three-way valve 28 into the pipe line 30 in a direction shown byan arrow G and further flows through the pipe line 30 into the secondcooling water chamber 22 of the suction side housing 2 to be finallydischarged from the cooling water discharge line 31.

In the heavy process, regarding the three-way valve 28, the switchingport 28 b is closed and the inlet port 28 a open.

Thereby, the cooling water flows sequentially through the cooling waterchamber 19 of the discharge side housing 3, the cooling water outletpipe 27, and the inlet port 28 a of the three-way valve 28 in adirection shown by an arrow H into the pipe line 30.

When the temperature of a water filling the first cooling water chamber7 of the main housing 1 becomes more than 100° C. due to heat generatedby compression of a gas in the inner cylinder 1 a, vaporization of thewater increases the inside pressure of the first cooling water chamber7. The pressure increase discharges partially the water into the coolingwater chamber 19 of the discharge side housing 3 through the coolingwater passage 26 (see FIG. 3) to mix it with a cooling water filling thedischarge side housing 3.

In this state, operating the flow control valve 32 toward its closedposition to increase the pressure drop therein causes that the vaportemperature in the first cooling water chamber 7 becomes higher than100° C. to increase the temperature of the main housing 1.

By heating the main housing 1 up to a temperature of 100° C. to 120° C.,a gas discharged from the main housing 1 is adjusted to have atemperature of around 350° C.

Since the vaporization temperatures of NH₄Cl, AlCl₂, etc. are lower than350° C. (under a pressure of 760 Torr), the deposit accumulation is notinitiated in the main housing 1, preventing a shutdown of the vacuumpump due to a piled-up deposit therein.

Furthermore, the control of the flow control valve 32 can prevent anexcessive temperature increase of the main housing 1. This eliminatesthe reduction of a service life of the vacuum pump and a fear of burnsdue to an excessive temperature increase of the main housing 1.

A temperature sensor (not shown) is provided in the first cooling waterchamber 7 of the main housing 1 for detecting whether an insidetemperature of the first cooling water chamber 7 is higher than apredetermined value. In addition, a warning device is provided forgiving an alarm based on a detected signal of the temperature sensor.Thereby, an operator who has heard the alarm operates the flow controlvalve 32 to control the temperature of a gas discharged from the mainhousing 1 to be around 350° C.

A control device for automatically controlling an opening/closingmechanism of the flow control valve based on a detected signal of thetemperature sensor may be provided, which eliminates the manualoperation of the flow control valve.

Industrial Applicability of the invention

The present invention, which is constituted as described above, hasoperational effects as described hereinafter:

(1) The dry vacuum pump according to the present invention is commonlyused in a light process and in a hard process by switching of thethree-way valve.

(2) When the vacuum pump is used in a hard process, the flow controlvalve is controlled in the open degree thereof to adjust the backpressure of the cooling water so that the temperature of the mainhousing is controlled.

Controlling the casing temperature to be at an adequate value canprevent the pileup of deposits and can prevent an excessive temperatureincrease of the vacuum pump.

What is claimed is:
 1. A vacuum pump comprising an inner cylinderaccommodating a pair of screw rotors engaging with each other, a suctionport communicating with one side of the inner cylinder, a discharge portcommunicating with another side of the inner cylinder, a main housinghaving an outer wall on which a first cooling water chamber is provided,a suction side housing attached on one end of the main housing andhaving an outer wall on which a second cooling water chamber isprovided, and a discharge side housing attached on another end of themain housing and having an outer wall on which a third cooling waterchamber is provided, wherein the third cooling water chamber of thedischarge side housing communicates with the first cooling water chamberof the main housing through a cooling water passage, and the thirdcooling water chamber of the discharge side housing has a cooling wateroutlet pipe which is connected to an inlet of a three-way valve, saidthree-way valve having a switching port which can communicate with thefirst cooling water chamber of the main housing, said the three-wayvalve having an outlet which is connected to the second cooling waterchamber of the suction side housing, the second cooling water chamberbeing connected to a cooling water discharge line provided with a valve.2. The vacuum pump set forth in claim 1 wherein the vacuum pump has atemperature sensor for detecting whether a temperature of the mainhousing becomes more than a predetermined value and a warning device forwarning of the open degree of the valve based on a sensed signal of thetemperature sensor.
 3. The vacuum pump set forth in claim 1 wherein thevacuum pump has a temperature sensor for detecting whether a temperatureof the main housing becomes more than a predetermined value and awarning device for warning of the open degree of the flow valve based ona sensed signal of the temperature sensor.
 4. The vacuum pump set forthin claim 1 wherein said valve provided in the cooling water dischargeline is a flow control valve.
 5. The vacuum pump set forth in claim 4wherein the vacuum pump has a temperature sensor for detecting whether atemperature of the main housing becomes more than a predetermined valueand a warning device for warning of the open degree of the valve basedon a sensed signal of the temperature sensor.
 6. The vacuum pump setforth in claim 4 wherein the vacuum pump has a temperature sensor fordetecting whether a temperature of the main housing becomes more than apredetermined value and a control device for automatically controllingthe open degree of the flow control valve based on a sensed signal ofthe temperature sensor.